Localization of a 42-kDa inositol 1,3,4,5-tetrakisphosphate receptor protein in retina and change in expression after optic nerve injury

The mRNA and protein expression of a 42-kDa inositol 1,3,4,5-tetrakisphosphate receptor (InsP(4)R) was investigated in cryostat and paraffin sections from rat, porcine and bovine retina. InsP(4)R mRNA was localized by in situ hybridization in the ganglion cell layer, the inner nuclear cell layer and the outermost part of the outer nuclear cell layer. For immunocytochemistry, we used an antibody raised against a 19-amino-acid peptide (peptide-3) derived from previous microsequencing of proteolytic fragments of the porcine InsP(4)R (Stricker et al., FEES Lett., 370 (1995) 236). The distribution of immunoreactivity was similar in all species investigated. Two cell types, most likely wide-field amacrine and retinal ganglion cells, were intensely stained. Prominent immunoreactivity in the on/off sublaminae of the inner plexiform layer and in the optic nerve layer indicates a pre- and/or post-synaptic localization of the protein. Moreover, significant InsP(4)R protein expression in the inner segment of photoreceptors points to a putative role of the second messenger InsP(4) in signaling processes related to phototransduction. However, also the endfeet of Muller glia cells in the optic nerve layer were intensely stained. Optic nerve crush caused only minor changes in retinal InsP(4)R mRNA levels whereas InsP(4)R immunoreactivity was attenuated for more than 4 weeks in the photoreceptor inner segments, wide field amacrine cells, and in retinal ganglion cells. The immunopositive sublaminae of the inner plexiform layer appeared to have shrunken. However, the signal intensity gradually recovered after 10 weeks. Since in parallel sections stained with a monoclonal antibody directed against the vesicular protein synaptophysin no changes were found, the alterations in InsP(4)R immunoreactivity induced by nerve injury are not due to a general decline in the expression of pre-synaptic proteins. We, therefore, hypothesize that the InsP(4)R might be linked to altered intracellular Ca2+ signaling after neuronal injury.